Hello

GigabitEthernet0/0/0 connects to what I believe is an ISP modem, but I'm admittedly unsure; that box connects via fiber to the ISP, from whom we received the specific gateway to connect to.

Can you confirm if that /24 subnet is a rfc 1918 addressing? - if it is then you have an upstream device performing NAT if it isn’t and the given your above statement, looking at the arp table you are confirming have /24 public wan subnet which is quite unusual to have unless you are large company hence the query regards the wan addressing - are you directly connected to the isp modem or another device that resides in between?

From this rtr can reach your internal hosts and from the internal host can they ping the rtr own wan ip address.

Would suggest also to remove the current default static route and reapply also stating the physical interface this will negate any recursive lookup

ip route 0.0.0.0 0.0.0.0 gig0/0/0 x.x.x.1